1. Field of the Invention
The present invention relates to a magnetic resonance spectroscopy (MRS) system for obtaining magnetic resonance (MR) spectrum information of specific atomic nuclei existing within an object under examination utilizing a magnetic resonance phenomenon.
2. Description of the Related Art
In the magnetic resonance imaging (MRI) system, a uniform static magnetic field is applied to a predetermined portion of an object under examination. Furthermore, to the object applied are a gradient magnetic field orthogonal to a slice portion including a portion of interest and an excitation pulse which is a high-frequency (usually a radio frequency) magnetic field orthogonal to the static magnetic field. The gradient magnetic field and the excitation pulse are superimposed on the static magnetic field. As a result, magnetic resonance is induced in specific nuclear spins within the slice portion only. With the system, after the removal of the high-frequency magnetic field, a magnetic resonance (MR) signal is detected from the resonant nuclei, and MR data based on the MR signal is processed to form a magnetic resonance (MR) image.
In a conventional system, as shown in FIG. 1, it was usual to obtain an MR signal for a slice S of an object P under examination and to form an MR image for the slice S. Recently, on the other hand, extracting an MR signal from only a local region S1 including a target portion in the slice S shown in FIG. 1 for subsequent spectrum analysis and acquiring MR spectrum information mirroring information on the density of specific nuclear spins and chemical shifts have been demanded especially in the field of medical diagnosis. Such a system as acquiring MR spectrum information is called the MRS system and incorporated in an MRI system, for example.
To detect the MR signal of such a local region only, two types of method have been used.
According to the first method, a portion of a uniform magnetic field Bo' is locally created in a static magnetic field Bo as shown in FIG. 2, and the MR phenomenon is caused to occur only in the portion to detect an MR signal. The method is referred to as the topical magnetic resonance (TMR) method.
The second method is called the surface coil method. When a relatively small coil, called a surface coil, is disposed over the surface of a target portion of an object under examination, a phenomenon occurs in which the sensitivity D (in the direction of depth) for a received MR signal detected by the coil becomes maximum (sensitivity D1) at the center of the x axis as shown in FIG. 3. Thus, the second method acquires partial MR information by detecting the MR signal from the central region.
However, these conventional methods involve problems to be solved. That is, in the case of the TMR method, in order to alter the distribution of the static magnetic field, the range of current applied to a coil and/or the position of the object under examination must be changed. This will result in complexity of the system. The surface coil method has considerable effect in receiving the MR signal from the surface and its vicinities of an object under examination, but cannot be applied to deep portions of the object.